In a group at high risk for CAD, we hypothesize that non-invasive testing will accurately detect the presence of occult asymptomatic CAD and thereby identify individuals at higher risk for future CAD events, such as sudden death, myocardial infarction and unstable angina. Coronary angiography will reveal significant flow-limiting coronary stenoses in most individuals with abnormal treadmill tests or thallium scans but in others, angiographic lesions in the proximal coronary arteries will be relatively mild and exercise-induced ischemia will be explained by reduced coronary flow reserve or impaired endothelium-dependent coronary artery dilatation.